I don't think it's showing up for anybody yet. Jake posted the link to show that Steam already carries all of Telltale's other downloadable games, so it's reasonable to assume that Puzzle Agent will show up there sometime closer to release.

Oh hey, you know how I said "unless otherwise stated?" So apparently there were conversations going on that neither Jake nor I knew about, and the Steam version won't be launching until later next month. Sorry to have misinformed you!